Basically, rotating memory includes at least one disk capable of storing magnetic data. A magnetic device that includes a gap typically is flown over the surface of the magnetic disk. Current is passed through coils in the magnetic device to produce magnetic lines of flux at the gap of the magnetic device which in turn magnetizes portions of the disk surface. An actuator arm includes the magnetic device and is used to move the magnetic device to various positions over the surface of the disk.
The magnetic device is also used to sense the magnetized portions of the disk. This is commonly called reading the data from the disk. The actuator arm moves the magnetic device to a selected area of interest that contains data needed for a particular computation by a computer. The magnetized portion of the disk produces flux lines or a magnetic field near the surface of the disk. As the magnetic device is flown or passed near the surface of a spinning disk, a voltage is induced within the coils of the magnetic device by the changing magnetic field generated by the rotating disk. This voltage is used to detect transitions in the magnetic field on the surface of the disk. These transitions represent the data stored on the disk.
In some instances, an error in the data read from the disk is detected which in turn triggers some corrective action. An error detected while the data is being read from the disk is commonly referred to as a read error. A soft read error is an error that is possible to correct. In many instances, the correction of the read error is handled without interrupting the computer system which is beyond the rotating disk storage device. The soft read error would also be corrected before the user could become aware of a soft read error.
In all instances,when a read error is encountered, a multistep procedure is attempted called a data recovery procedure. When the steps in the data recovery procedure are unable to correct a read error, then the read error is termed a hard error.
Hard errors mean data has been lost. Once data is read with a high DRP count or lost from a particular portion of a disk such as a sector, the area is reallocated to another spare magnetizable portion on the disk drive. During the process of reallocation, errors may be recovered. Other hard errors may be ultimately recoverable but only after returning the disk drive to the factory. This procedure is time consuming for the people or the system relying on a disk drive and is also expensive for the manufacturer of the disk drive. Thus, sending a disk drive back to the factory to recover errors is usually done only in the rare instance when large amounts of important data are lost. Since read errors are undesirable, there always is a need for any step, process or apparatus which enhances a disk storage device's ability to recover any read errors without allowing them to become hard. Any enhancement in the ability to recover read errors means that the rate of occurrence of hard errors will be reduced which in turn minimizes any loss of data inconvenience for customers relying on the disk drive, and expensive data recovery at the factory.
In the past, various steps have been used to recover data with data recovery procedures. U.S. Pat. No. 4.821.125 issued to Christensen et al. discloses a data recovery procedure at column 8. line 32 and following. The data recovery procedure is also depicted in FIG. 6 of that patent in flow chart form. Basically, the Christensen et al. patent teaches rereading the portion of the disk containing the error several times and then changing a channel characteristic and then again rereading the portion of the disk several times in an attempt to recover data which is in error. The disk is first reread without error correction code. After this certain channel characteristics are changed. The portion of the disk is reread with error correction code, then the head is offset from the track both inwardly and outwardly, then a reread is attempted after changing the variable delta-V detection parameter, and then a second error correction code is used during a reread. The Christensen et al. patent discloses one particular error recovery procedure and the specific step of changing the variable delta-V to recover errors.
There are many different error recovery procedures that feature all sorts of steps other than those shown in the Christensen et al. patent. Other steps that are known in the art of data recovery include electronically varying the timing window used to detect transitions. The timing window can be shifted forward or backward from its on center position to detect transitions which occur either slightly ahead or slightly behind the time which they are supposed to occur in the window.
Another data recovery procedure for recovering read errors is disclosed in U.S. Pat. No. 4,516,165 issued to Cunningham et al. and entitled "Error Recovery Procedure Using Selective Erasure". The Cunningham et al. patent discloses reading and storing the data on the two adjacent tracks on either side of the track containing the error. The adjacent tracks are then each erased and the track containing the error is reread in an attempt to recover the error. This technique allows recovery of data from a track which exhibits consistent errors either because the track was partially occluded by adjoining tracks through writing over a portion of the track of interest due to track misregistration or the data on adjoining tracks was phased and of such a frequency that the lateral readback amplitudes were excessive.
The techniques for data recovery listed above are useful for certain applications. As mentioned previously, there is always a need for a step or apparatus which enhances the ability of a disk drive or magnetic storage unit in recovering data in a track or sector that is difficult to read.